Handover is a vital part of any mobile communication system. A handover is the process of transferring an ongoing connection of a User Equipment, UE, from one cell, i.e. the serving cell, to another cell, i.e. the target cell, in order to accomplish a transparent service over a larger area. The handover should happen without any loss of data and with as small interruption as possible.
To enable a handover, it is necessary to find a suitable target cell and to ensure that it is possible to sustain reliable communication with that target cell. To make sure that it is possible to sustain reliable communication with the target cell, the connection quality in the target cell needs to be estimated before the handover can take place.
The quality in the target cell is estimated by measurements related to the UE. Both downlink and uplink measurements can be considered. In legacy systems, handover based on downlink measurements has been the most common solution; a natural solution as all base stations continuously transmit pilot signals that UEs in neighbor cells can use to estimate the target cell quality. This is true in GSM, BCCH, WCDMA, CPICH and LTE, CRS as well as in WiFi, beacon. This makes it possible to estimate the quality of neighbor cells with relatively good accuracy.
Modern cellular networks will use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow beams to increase the signal strength in some directions and/or to reduce interference in other directions. Continuously transmitting pilot signals in all these beams is then less attractive, since it will generate a lot of interference and also increase the base station energy consumption. During a handover, maintenance of good Signal to Noise Ratio, SNR, and high bit rates require that a UE is handed over from one beam to another. In addition, in the high frequencies considered for 5G systems the radio propagation properties, e.g. low diffraction and poor penetration, the suitability of a certain beam may be quite sensitive to rather small movements and even rotations of the UE. Hence, which beam to hand over a UE to may not be easily determined and to support handover between beams, the UE has to perform a beam finding procedure. During such a beam finding procedure the radio access network nodes that are potential target network nodes for the handover, i.e. candidate access network nodes, transmit Downlink beams, DL beams, e.g. identified by downlink signals, e.g. synchronization and/or reference signals, for the UE to measure on. The beams are typically sequentially transmitted in a manner usually referred to as a beam sweep. The beam sweep may be continuously repeated or activated on demand. The UE searches for the signals transmitted in the beams in the beam sweep and measures their respective quality. The beam with the best measured quality is typically selected as the target for the handover. As mentioned above, system based on narrow beams is sensitive to small movements and rotations of the UE and hence the radio link quality may deteriorate fast, even to the point where the link is lost. This increases the need for a fast handover procedure in beam based system, which is challenging when the handover procedure includes a beam finding procedure involving several beams. Furthermore, with a large number of beams, the above beam-sweeping procedure becomes complicated, which motivates the introduction of alternative solutions.
In a system with advanced antennas, it therefore becomes attractive to rely on uplink measurements, UL measurements. Cellular systems may rely on uplink measurements already today: the UE transmits some uplink signal and several network nodes measure on that signal. The uplink signal may e.g., be a sounding signal, a reference signal or a combined synchronization and reference signal.
One reason that makes the UL measurement based handover an attractive alternative in systems with advanced antennas, i.e. antennas that are capable of and, in high frequencies, relying heavily on advanced beamforming, is the difference in the UL and DL link budget. Since the access network node typically has more antennas and more advanced antenna configurations and receiver than the UE, the receiver gain in the access network node is higher than in the UE, making the link budget more favorable in the uplink. For this reason, beamforming of the transmission of the uplink signal used for UL measurement based handover is not as crucial as for DL measurement based handover, using DL beam sweeps as described above. Typically, a single omnidirectional uplink signal transmission, or possibly a beam sweep consisting of a few wide UL beams, suffices to reach and provide a measurement opportunity for all the candidate access network nodes, since the beamforming gain is provided by the AN. This is clearly more resource efficient than transmitting multiple DL signals in different beam directions from different candidate access network nodes. Furthermore, an additional benefit of UL measurement based handover compared to DL measurement based ditto is that the base station/access network node receiving the uplink signal from the UE may estimate the DL quality based on UL/DL reciprocity especially in TDD deployments which are expected to prevail in high frequency 5G systems. In addition to the downlink quality, also, the DL beam direction to use can be estimated.
The UL measurement based handover starts by initiating the UE uplink signal transmission, so that the candidate cells/access network nodes can measure on it. The measurements of the quality of the UL signal transmitted from all UEs are collected and compared, the network compares all the collected measurements and decides on a suitable target cell/access network node and the decision is communicated to the UE.
UL measurement based HO requires that all candidate access network nodes, candidate ANs, are prepared to listen when the UE transmits its UL signal, e.g. USS.
However, a source AN cannot freely allocate any time and frequency resource for the UE to transmit an UL signal which the candidate AN are supposed to listen for. An arbitrary time/frequency resource may be occupied/reserved for other tasks in a candidate AN, such as reception of UL data from an already connected UE, reception of an UL signal from another UE for which handover is considered, transmission of statically repeated signals, e.g. reference signals, synchronization signals or system information, or transmission of a downlink signal when the system operates in Time Division Duplex Downlink transmit mode, TDD DL TX mode. Brute force enforcement of the UL slot selected by the source node may therefore compromise other important functions in the candidate nodes.
Thus, a mechanism enabling fast allocation of UL transmission resources in UL measurement based handover with limited or no conflicts in the UL signal reception is needed. More specifically, mechanisms are needed that allow for an efficient handling of a handover procedure based on UL measurement procedures.
The present technology aims to provide such mechanisms.